1. Field of the Invention
The present invention relates to liquid crystal displays using ferroelectric liquid crystal, more specifically, liquid crystal displays in which a photo alignment film is used to control the orientation of ferroelectric liquid crystal.
2. Description of the Related Art
Since liquid crystal displays have features that it is thin and is low in power consumption and other features, the use thereof has been expanding in various articles from large-sized displays to portable information terminals and the development thereof has been actively made. Conventionally, for liquid crystal displays, a TN system, an STN multiplex driving system, an active matrix driving system in which thin film transistors (TFTs) are used in TN, and other shave been developed and made practicable. However, nematic liquid crystal is used therein; therefore, the response speed of the liquid crystal material is as small as several milliseconds to several tens of milliseconds and it cannot be said that these sufficiently cope with display of moving images.
Ferroelectric liquid crystal (FLC) exhibits a very short response time in order of microseconds, and thus FLC is a liquid crystal suitable for high-speed devices. About ferroelectric liquid crystal, there is well known a bistable liquid crystal which has two stable states when no voltage is applied thereto and is suggested by Clark and Lagerwall (FIG. 1). However, the liquid crystal has a problem that the liquid crystal has memory property but graduation display cannot be attained since the switching thereof is limited to two states, namely, bright and dark states.
In recent years, attention has been paid to ferroelectric liquid crystal in which the liquid crystal layer thereof is stable in a single state (hereinafter referred to as “monostable”) when no voltage is applied thereto as a liquid crystal making it possible to attain graduation display by the matter that the director (the inclination of the molecule axis) of the liquid crystal is continuously changed by a change in applied voltage so as to analogue-modulate the light transmission thereof (NONAKA, T., LI, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599., FIG. 1). As such liquid crystal exhibiting mono-stability, there is usually used a ferroelectric liquid crystal in which phase change is caused between chorestric phase (Ch) and chiral smectic C phase (SmC*) via no smectic A phase (SmA). When ferroelectric liquid crystal exhibits mono-stability in this manner, the liquid crystal does not have any memory property and it is desired to drive the liquid crystal through an active matrix system in which an active element such as a transistor or a diode is added to each pixel. In the case of using, in particular, an active matrix system using TFT elements as active elements, high-quality display can be attained since target pixels can be certainly switched on and off.
In the mean time, ferroelectric liquid crystal has a higher order of molecules therein than nematic liquid crystal; therefore, the former liquid crystal is not easily oriented so that defects called zigzag defects or hairpin defects are easily generated. Such defects cause a fall in contrast based on light leakage. In particular, ferroelectric liquid crystal undergoing phase transition via no SmA phase generates two domains different in the layer normal-line direction thereof (hereinafter referred to as “double domains”) (FIG. 2). The double domains give such display that black and white are reversed so as to cause a serious problem (FIG. 3). As the method for removing the double domains, known is an electric field induced technique (, which uses DC voltage during cooling process) of heating a liquid crystal cell to a temperature not lower than the Ch phase thereof, and then cooling the liquid crystal cell slowly while applying a DC voltage thereto (PATEL, J., and GOODBY, J. W., 1986, J. Appl. Phys., 59, 2355). This method has problems that the orientation of the liquid crystal is disturbed when the temperature thereof is again raised to a temperature not lower than the phase transition temperature thereof and the orientation is disturbed in regions where no electric field acts between pixel electrodes, and other problems.
As the technique for subjecting liquid crystal to orienting treatment, there is known a method of using an orientation film. The method is classified into the rubbing method and the optical orienting method. The rubbing method is a method of subjecting a substrate coated with a polyimide film to rubbing treatment to orient chains of the polyimide polymer in the direction of the rubbing, thereby orienting liquid crystal molecules on the film. The rubbing method is excellent in controllability of the orientation of nematic liquid crystal, and is generally an industrially applicable technique. However, this method has the following problems: the generation of static electricity and dust, unevenness in the power for regulating the orientation and the tilt angle of liquid crystal, treatment-ununiformity caused when a large area is treated, and soon. Thus, this method is unsuitable as a method for orienting ferroelectric liquid crystal, in which orientation defects are easily generated. Moreover, the rubbing method cannot overcome double domains.
The above-mentioned optical orienting method is a non-contact orienting method instead of the rubbing method. The optical orienting method is a method of radiating light the polarization of which is controlled onto a substrate coated with a polymer film or a monomolecular film to generate photo-excited reaction (decomposition, isomerization or dimerization) so as to give anisotropy to the polymer film or the monomolecular film, thereby orienting the liquid crystal molecules on the film. This method is useful since the generation of static electricity or dust, which is a problem in the rubbing method, does not arise and the orientating treatment can be quantitatively controlled. However, even if this method is used, it is difficult to suppress the generation of double domains and obtain mono-domain alignment.
Another method for making ferroelectric liquid crystal monostable is the polymer stabilization method. The polymer stabilization method is a method of injecting a ferroelectric liquid crystal blended with an ultraviolet curable monomer into a liquid crystal cell subjected to orienting treatment, and then radiating ultraviolet rays onto the liquid crystal in the state that an AC or DC voltage is applied thereto, thereby polymerizing the monomer to stabilize the liquid crystal. However, the method has problems that the production process thereof is complicated and the voltage for driving the liquid crystal is high.
As still another method for giving mono-domains, Japanese Patent Application Laid-Open (JP-A) No. 2003-5223 describes a method of subjecting one out of orienting-films on and beneath a ferroelectric liquid crystal to rubbing treatment, and subjecting the other to optical orienting treatment, thereby orienting the ferroelectric liquid crystal. However, according to this method, there remain problems such as the generation of static electricity or dust and treatment-ununiformity generated when a large area is treated, as described above, since only one of the films is rubbed.
In recent years, color liquid crystal displays have been actively developed. The method for realizing color display is generally classified into a color filter system and a field sequential color system. The color filter system is a system of using a white light source as a back light and attaching a micro color filter in R, G or B color to each pixel, thereby realizing color display. On the other hand, the field sequential color system is a system of switching a back light into R, G, B, R, G, B . . . with time, and opening and shutting a black and white shutter of a ferroelectric liquid crystal in synchronization therewith to mix the colors with time by afterimage effect on the retina, thereby realizing color display. This field sequential color system makes it possible to attain color display in each pixel, and does not require any color filter low in transmission. As a result, this system is useful since the system is capable of attaining bright and highly precise color display and realizing low power consumption and low costs. However, the field sequential color system is a system in which each pixel is subjected to time sharing; it is therefore necessary for the liquid crystal as the black and white shutter to have high-speed responsiveness in order to give a good moving image display property. If ferroelectric liquid crystal is used, this problem can be solved. However, the ferroelectric liquid crystal has a problem that orientation defects are easily generated, as described above, and thus the color system using this liquid crystal has not been made practicable.